Having been persuaded to go through with the headline fight by his mother after a row over the ring walks, Chisora looked set for one of the biggest wins of his career when he landed his signature looping overhand right moments after the first bell.
Parker weathered some more punishing moments before finishing strongly against his tiring rival and was given the nod 116-111 and 115-113 by two of the judges, with a third scoring the contest 115-113 in favour of Chisora.
Chisora felt he had done enough to have his hand raised and lamented another narrow decision loss, as he said afterwards: "I'm beyond getting upset now. It's just difficult.
"I train hard, I fight, I put the pressure on. I bring everything and this is the treatment I get from boxing. This is unbelievable. Most people (thought) I won the fight."
The close nature of the result could lead to calls for a rematch while victory for Parker, his 29th in 31 professional contests, inches him closer to another world title shot.
The former WBO champion has recently switched trainers to Tyson Fury's coach Andy Lee, while Chisora (now 32-11, 23KOs) had hired the renowned Buddy McGirt to be his chief cornerman for the contest.
The build-up had been largely serene – including the fighters eating breakfast together on Thursday – but after Friday's weigh-in Chisora threatened to pull the plug if he did not get to walk to the ring second.
A coin toss went in Parker's favour, and he chose to come out second. Chisora's manager David Haye revealed the situation was resolved on Saturday morning after intervention from the 37-year-old's mother Viola.
Perhaps spurred on by the events of the preceding 24 hours, Chisora put Parker on his knees with the first punch of the fight.
Parker had never been stopped in his professional career and this seemed more of a flash knockdown than anything Chisora could follow up to end the fight early.
He looked on menacingly after being ushered to a neutral corner but Parker rose to his feet and looked to avoid the vaunted right hand of his opponent. While he was forced to wear another big shot in the third round, this time he stayed upright.
Slowly but surely Parker showed signs of resurgence after a difficult first few rounds, taking advantage of Chisora slowing down to pepper the British veteran with a combination that backed him up against the ropes in the seventh.
The pair engaged in several enthralling toe-to-toe exchanges in the eighth, showing tremendous heart, with both men unwilling to take a backward step.
It was Parker who was getting on top down the stretch and a punishing right hand seemed to wobble Chisora, who survived to the final bell but was left lamenting his second successive defeat – having been beaten by former undisputed cruiserweight champion Oleksandr Usyk last October.